1. Field of the Invention The present invention relates generally to vehicle charging systems, and more specifically to controlling the output of an alternator using alternator temperature feedback.
2. Background
A typical prior art charging circuit 10 is depicted in FIG. 1. The charging system provides electrical energy while the engine is running to recharge the battery and to power electrical devices. As seen in FIG. 1, a battery 12 is connected between a ground 14 and a positive or "hot" lead 16, which leads to the electrical systems (not shown) of the vehicle and to the alternator 18. This lead 16 is a path for current out of the battery during undercharging or discharging, and a path for current into the battery during charging. The alternator 18 is typically driven by a pulley 20, which is driven by a belt (not shown) from the prime mover or engine (not shown). The electrical systems of the vehicle are powered through lead 22. An ignition switch 24 is also connected to the hot lead 16. Typically, an indicator lamp 26 is present to indicate a discharge state. A voltage regulator input lead 28 connects to the voltage regulator 30, which determines the output of the alternator by controlling the excitation voltage provided to the field winding of the alternator via line 32, as will be discussed in greater detail below.
The basic layout of a vehicle alternator is well known. An alternator is typically a three-phase AC generator that typically comprises a rotor, which is essentially a spinning magnetic field which is turned by the vehicle's engine, and a stator, which is a stationary output winding. The operation is based on Faraday's law of electromagnetic induction. As the rotor is moved creating a varying magnetic field, electromotive force, or EMF, is induced in the windings resulting in current output. In order to produce a magnetic field in the rotor, field windings in the rotor are connected to a source of excitation current. The output from each of the three phases is AC, which is then rectified into DC through a rectifier bridge.
One major byproduct of electrical generation is heat. As the output power of the alternator increases, which typically increases with load, the heat generated by the alternator increases. If the alternator is placed in a condition of excessive loading with insufficient ventilation, permanent damage can occur to the alternator components, especially the electronics and the diodes or rectifier elements. The risk is especially great at vehicle idle speed, when the rotation speed of the alternator governs both its ability to efficiently generate adequate power, and ventilation is at a minimum. Many modern driving environments involving long idle periods in traffic, and the increasing number of electrically powered convenience accessories in vehicles place exceptional demands on the alternator.
Before going into further detail about the temperature effects on the alternator, a discussion of the charging system will be presented.
A prior art rectifier bridge is depicted in FIG. 2. Prior art rectifier bridges for vehicular alternators have typically used an arrangement of diodes D1-D6, which serve as electrical check valves. Each of the three phases V1-V3 is connected to two diodes, such that the negative and positive AC output of each phase are each rectified into DC voltage. FIG. 3 depicts the output that results from the prior art diode bridge, as is well known to those skilled in the art.
The prior art rectifier bridge is very dependable and has been used successfully in claw pole, synchronous (Lundell) alternators in vehicles for decades. However, as discussed above, the modern era has placed increasing power demands on vehicle electrical systems through the constant addition of new electrical and electronic accessories, control systems, etc. to the vehicle. It has also become increasingly less desirable to solve these power requirement problems through increasing the size of the alternator, as available space in the hood of the modern vehicle is densely packed. Further, the additional engine power required to turn a larger alternator decreases overall efficiency of the vehicle, as does the additional weight.
A result of the increasing demands placed on the alternator is that often when the engine is at an idle, when alternator speed and hence power output efficiency are quite low, a deficit in the charging system results. In such situations, the battery supplies the required energy in a discharging state. Repetitive charging and discharging of batteries used in vehicles, typically lead-acid storage batteries, leads to shorter longevity, which is undesirable both from the point of view of the consumer and the environment.
One limitation of the diode is that it is not controllable in the sense that its switching points are inherent in its design. The prior art contains other arrangements, which replace the diodes of a conventional rectifier or inverter bridge with controllable elements, such as transistors or thyristors.
The voltage regulator, a major part of the charging system, controls the output of the alternator by controlling the excitation current in the field windings. By changing the excitation current in the field windings in the rotor, the strength of the magnetic field of the rotor is affected, and thus the output of the stator windings of the alternator. Prior art voltage regulators typically are preset to maintain the charging voltage of the alternator at a predetermined point, typically between 13 and 15 V. In an automotive charging system, in order for the battery to recharge, the output voltage of the alternator must be higher than that of the battery. However, a large difference can overload the battery, causing electrochemical damage, which decreases its longevity. For this reason, only a small potential difference above the typical 12 to 12.6 V of a fully charged battery is used. Because the rotational speed of the alternator varies with engine speed, the voltage regulator is necessary to maintain the voltage of the alternator output.
A prior art electronic voltage regulator is depicted in FIG. 4. By placing resistances in and out of series with the excitation field current (which is supplied in most cases by the battery), the strength of the excitation field in the rotor can be modified. In this embodiment, if the alternator speed is too low and/or the electrical load too high, the regulator will compensate so that the alternator achieves a preset voltage output. Other systems vary the duty cycle of the field windings in order to arrive at a preset voltage output. During periods of low output, the duty cycle could be as low as 10%.
It should be noted that the prior art rectifier bridge and voltage regulator run as essentially discrete systems, with the voltage regulator determining the alternator output through varying the current through the excitation field winding. Accordingly, the prior art approaches alternator output control exclusively through the field controller. To prevent over-temperature operation of the alternator, a simple solution is to place a thermal breaker in the field controller so that when an over temperature condition is detected, the excitation field winding is turned off, thus discontinuing power generation. When the temperature decreases to an acceptable level, the breaker resets and the alternator begins generating electricity once again. Of course, while the alternator is off line, the battery alone is responsible for providing electricity to the vehicle, which leads to battery discharge.
By allowing repetitive discharge of the battery under periods of high loads, the battery itself is shortened in its longevity. The end result is more dead lead-acid batteries in circulation, which is clearly disadvantageous from system reliability, maintenance cost, and environmental perspectives.
Finally, it should be understood that overall cost is an extremely important criterion in evaluating vehicular design solutions.